Input buffers are commonly used in a wide variety of digital integrated circuits. For example, input buffers are commonly used to couple command signals, address signals, write data signals and clock signals from externally accessible terminals to internal circuits of integrated circuit memory devices such as dynamic random access memory (“DRAM”) devices.
A typical input buffer 10 used in integrated circuits such as memory devices is shown in FIG. 1. The input buffer 10 includes a first inverting buffer circuit 14 receiving complementary external clock signals XCLK and XCLK*. The buffer circuit 14 outputs a digital signal having a logic level depending upon the polarity of a comparison between the XCLK and XCLK* signals. An output of the buffer circuit 14 is applied to an input of a second inverting buffer circuit 18, which provides additional gain for the XCLK and XCLK* signals. Another input of the buffer circuit 18 receives a DVC2 signal, which generally has a magnitude equal to one-half of a power supply voltage VCC applied to the buffer circuits 14, 18. In such cases, the logic levels output by the buffer circuits 14, 18 generally transition between zero volts and the power supply voltage VCC. The buffer circuits 14, 18 may also be selectively enabled by an enable signal EN applied to respective enable inputs of the amplifiers 14, 18.
The output of the buffer circuit 18 is applied to the gate of a PMOS transistor 20 and the gate of an NMOS transistor 24. The source of the PMOS transistor 20 receives a supply voltage VCC, while the source of the NMOS transistor 24 is connected to ground. The drains of the transistors 20, 24 are connected to each other and to an output terminal 26 through an inverter 28.
In operation, when the magnitude of the XCLK signal is greater than the magnitude of the XCLK*signal, the buffer circuit 14 outputs a low logic level, and the buffer circuit 18 outputs a high logic level. This high logic level turns OFF the PMOS transistor 20 and turns ON the NMOS transistor 24, thereby pulling in the input of the inverter 28 to ground. A high logic level is therefore produced at the output terminal 26.
When the magnitude of the XCLK signal is less than the magnitude of the XCLK*signal, the buffer circuit 14 outputs a high logic level, and the buffer circuit 18 outputs a low logic level. This low logic level turns ON the PMOS transistor 20 and turns OFF the NMOS transistor 24, thereby driving the input of the inverter 28 to the supply voltage VCC. The inverter 28 then outputs a low logic level at the output terminal 26.
Input buffers, including the input buffer 10 shown in FIG. 1, generally perform a number of advantageous functions. Input buffers generally provide a high input impedance to avoid unduly loading signal lines coupled to their inputs. They also condition signals applied to internal circuits so that internal signals have well defined logic levels and transition characteristics. Other advantages of input buffers are also well-known to one skilled in the art.
Although input buffers can provide a number of advantages, they are not without some disadvantages and limitations. For example, the time required to couple signals through input buffers can greatly increase the time required to couple externally applied digital signals to circuits within an integrated circuit. For example, each of the buffer circuits 14, 18 can significantly delay the propagation of digital signals coupled from their inputs to their outputs. This delay primarily results from capacitances inside the buffer circuits 14, 18. The digital signals also can be delayed in being coupled to the transistors 20, 24. This delay is primarily due to the time required for the digital signals to propagate through signal lines from the output of the buffer circuit 18 to the gates of the transistors 20, 24, which is affected by the length of the signal lines and their capacitances. Further delay can be encountered in the transistors 20, 24 and the inverter, which are again primarily due to be internal circuit capacitances in those devices.
The delays in coupling digital signals through input buffers, such as the input buffer 10, can significantly reduce the operating speed of integrated circuits, such as memory devices, using such input buffers. These delays become even more problematic as the operating speeds of integrated circuits continue to increase.
Various approaches have been used to increase the operating speed of input buffers. One approach relies on coupling positive feedback from downstream circuitry in the input buffer to upstream circuitry of the input buffer, such as its input. As it is well-known in the art, positive feedback increases the gain of the input buffer thereby causing the output signal to more quickly transition in response to a transition of an input signal. However, the amount of positive feedback must be relatively weak or else the input buffer will be bi-stable, i.e., the output signal may be latched at either of the two logic levels of the output signal regardless of the logic level of the input signal. While it might be possible to overcome this strong positive feedback, the need to drive the output signal with sufficient strength to overcome the positive feedback can itself result in significant delays. Furthermore, when the output is latched to a logic level that does not correspond to the logic level of the input signal, the positive feedback in effect becomes negative feedback and is therefore counterproductive and causing the output of the buffer to transition to the new logic level.
There is therefore a need for a digital signal input buffer and method that can couple digital signals from externally accessible terminals to internal circuitry with a minimum of propagation delay.